Hi, i have the player animations in .XSI format and i have the plugin to import .xsi files into max, but, i want to know if there is a way to modify the animations using character studio, or save the current ones as .bip files

You'd need to use SoftImage xsi and export as BHV I beleive. Which Max allows you to import onto bipeds. Another method is to use an app called I beleive Motion Capture Studio. It uses the FBX file format. It allows to import xsi and export as a couple different formats. Hope that helps.